The LIM domain is a cysteine-rich motif which was first defined in the proteins Lin-11 from C. elegans (Freyd, G. et al. (1990) Nature 344:876-879) , insulin gene enhancer binding protein ISL1 from rat (Karlsson, O. et al. (1990) Nature 344:879-882), and Mec-3 from C. elegans (Way, J. C. et al. (1 988) Cell 54:5-16). The name LIM is derived from the first letter of the names of these three proteins.
The sequence of the LIM domain is highly conserved among proteins found in different tissues and across a variety of species, which is an indication of the functional importance of this domain (Sanchez-Garcia, I. et al. (1994) Trends Genet. 10:315-320). Two main classes of LIM proteins are known. One class consists of proteins that, like Lin-11, ISL1 and Mec-3, contain two LIM domains plus a homeodomain and are thus designated LIM-HD proteins. The second class of LIM proteins consists of one or more LIM domains without a homeodomain and are thus designated "LIM-only" proteins.
A LIM domain is defined by a conserved consensus amino acid sequence (Wang, X. et al. (1992) J. Biol. Chem. 267:9176-9184). The domain consists of two adjacent zinc-finger motifs. Two zinc ions bind to a LIM domain, one per zinc finger. LIM domains function as protein-binding interfaces (Schmeichel, K. L. et al. (1994) Cell 79:211-219) and thus may act as cofactors in cell signaling.
Some LIM proteins exhibit oncogenic activity while other members of this diverse group may act as tumor suppressor molecules. The rhombotin genes (RBTN1 and RBTN2) encode LIM proteins which have been implicated in the control of the neoplastic phenotype. RBTN2, identified in childhood T cell acute lymphoblastic leukemia (Boehm, T. et al. (1988) EMBO J. 7:385-394), is essential for erythroid cell development. A homozygous null mutation in RBTN2 results in failure of yolk sac erythropoesis and embryonic death (Warren, A. J. et al. (1994) Cell 78:45-57). In transgenic mice, cell-specific overexpression of RBTN1 or RBTN2 results in the generation of acute lymphoblastic lymphomas at low frequency (Fisch, P. et al. (1992) Oncogene 7:2389-2397; McGuire, E. A. et al. (1992) Mol. Cell Biol. 12:4186-4196).
Human cysteine-rich protein (CRP) is widely expressed in a variety of tissues. Its expression is induced shortly after serum stimulation of fibroblasts in the G.sub.o growth-arrest phase of the cell cycle. The serum induction kinetics of CRP closely parallel those of the c-myc oncogene, which suggests that these two genes respond to the same regulatory pathways and may share transcription control features (Wang et al, supra). CRP is thus proposed to be a primary response gene induced as the cell transits from G.sub.o to G.sub.1 or progresses from the S phase of the cell cycle, and is proposed to act as an oncogene.
Muscle LIM proteins (MLP) are involved in regulating cell-specific gene expression in heart and skeletal muscle. The expression of human MLP and the Drosophila homolog DMLP1 coincide with the differentiation of myoblasts into muscle cells. Overexpression of MLP in C2 myoblasts potentiates myogenic cell differentiation, whereas expression of antisense MLP RNA retards myoblast differentiation (Arber, S. et al. (1994) Cell 79:221-231).
Smooth muscle LIM protein (smLIM) from rat is expressed preferentially in aortic smooth muscle cells. Like MLP, it is a developmentally regulated nuclear protein. SmLIM mRNA levels decrease in vivo in response to vessel wall injury during periods of maximal smooth muscle proliferation (Jain, M. K. etal. (1996) J. Biol. Chem. 271:10194-10199).
Reversion-induced LIM (RIL) protein from rat is highly expressed in fibroblasts and is down-regulated in H-ras transformed cells. Expression of RIL is restored in phenotypic revertants derived from H-ras transformed cells. RIL is thus proposed to be involved in the maintenance of normal cell growth (Kiess, M. et al. (1995) Oncogene 10:61-68). RIL is expressed in brain, heart, testes and variety of epithelia. The pattern of RIL protein expression suggests a physiological function in epithelial cells and in postmitotic neurons of the brain.
CLP36 protein is highly expressed in normal rat hepatocytes and is down-regulated in hypoxic hepatocytes. The relationship of this down-regulation to hypoxic injury is not understood. CLP36 is found in a wide variety of tissues, at high abundance in heart, lung and liver, moderate abundance in spleen and skeletal muscle, and at extremely low abundance in testis and brain (Wang H. et al. (1995) Gene 165:267-271).
The discovery of polynucleotides encoding LIM proteins, and the protein molecules themselves, presents the opportunity to investigate physiological processes relating to the control of cellular differentiation, proliferation, and response to tissue injury. Discovery of novel LIM proteins and the polynucleotides encoding them satisfies a need in the art by providing new diagnostic or therapeutic compositions directed toward diseases relating to cell damage and abnormal cell growth and proliferation such as arteriosclerosis and cancer.